Closes #5382: Adds bors toml

[sblatz]

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

After merge

• Milestone: Make sure issues finished by this pull request are added to the milestone of the version currently in development.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-io]

Codecov Report

Merging #5899 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #5899      +/-   ##
============================================
+ Coverage      14.3%   14.31%   +0.01%     
  Complexity      320      320              
============================================
  Files           257      257              
  Lines         10622    10624       +2     
  Branches       1553     1553              
============================================
+ Hits           1519     1521       +2     
  Misses         8977     8977              
  Partials        126      126

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/org/mozilla/fenix/BrowserDirection.kt	100% <0%> (ø)	2% <0%> (ø)	⬇️

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